It is just the screen not the system.. Just clean all screen and need dry screen..press lock and unlock button.. All phone body must dry all day. This is because of too sensitive screen even sweat,can causes lag
I noticed some lag initially too, like tapping on an email box from the main screen had like an extra second until the animation would start. It did gi back to normal over time. I've also rebooted the phone a few times since then (unrelated reboots). That could have helped.